Generally, secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries, lithium ion batteries and lithium ion polymer batteries. Such secondary batteries are classified into lithium based batteries and nickel-hydrogen based batteries. Lithium based batteries are mainly used for small products such as digital camera, P-DVD, MP3P, cellular phone, PDA, portable game device, power tool and E-bike, and nickel-hydrogen based batteries are mainly applied to and used for large products such as electric vehicle and hybrid electric vehicle, which need high output.
Meanwhile, for driving an electric vehicle or a hybrid electric vehicle, a motor should be operated, which requires high output. For this purpose, a battery employed in an electric vehicle or a hybrid electric vehicle uses electricity output from a group of plural unit cells connected in series or in parallel as its power source.
However, in case a plurality of unit cells are connected in a battery, there may be made differences among charge capacities of the unit cells after the unit cells are repeatedly charged and discharged. If charging/discharging is continued while such charge capacity different is left alone, some of unit cells may come into a overcharged or overdischarged state, which may disturb supplying stable power to a load (e.g., a motor).
To solve this problem, various circuits for continuously monitoring charge capacity of battery cells and then balancing the charge capacity of the battery cells to a constant level have been proposed and used to a battery management system.
For balancing charge capacities of battery cells, it is required to sense charge voltage of each battery cell. Conventionally, it was a common case that a circuit for sensing charge voltage of each battery cell and a circuit for balancing charge capacity of each battery cell are separately configured. However, if the charge voltage sensing circuit and the charge capacity balancing circuit are configured separately, each circuit should use a separate element individually, which causes various problems (e.g., increased cost, increased breakdown rate). Accordingly, there is an urgent need for a scheme to efficiently balance charge capacities of battery cells with a low cost using a simple circuit configuration.